Acoustic apparatus



Dec. 23, 1969 J. v. BOUYOUCOS ACOUSTIC APPARATUS 4 Sheets-Sheet 1 Original Filed Feb. 9, 1961 R. OUCOS INVEN JOH/V \l. 50

A 7 TOR/VE Y Dec. 23, 1969 J. v. BOUYOUCOS ACOUSTIC APPARATUS 4 Sheets-Sheet 2 Original Filed Feb. 9, 1961 I I I- Dec. 23, 1969 J. v. aouvoucos ACOUSTIC APPARATUS 4 Sheets-Sheet 5 Original Fileu Feb, 9 1961 Dec. 23, 1969 J. v. eouvoucos ACOUSTIC APPARATUS 4 Sheets-Sheet 4 Original Filed Feb. 9, 1961 INVENTOR JOHN V. BOUYOUCOS m/ m m,

MASTER GENERATW A TTOHNE Y United States US. Cl. 116137 2 Claims ABSTRACT OF THE DIS'CLQSURE Self-excited hydroacoustic generators are inter-connected so that acoustic energy is coupled therebetween. The generators are thereby synchronized and provide an array for projecting high power acoustic waves.

The present invention relates to acoustic vibration generators, and particularly to phase synchronism means for a plurality of acoustic vibration generators. This application is a division of my US. Patent No. 3,212,472, issued Oct. 19, 1965 Ser. No. 88,164 filed Feb. 9, 1961, for Acoustic Vibration Generator and Coupler.

In the above co-pending patent application there is described acoustic vibration generators of the self-excited type and acoustic vibrator generator coupling means employed in the vibration generators for coupling acoustic energy from the generators to a load. The coupling means for coupling acoustic energy from the oscillator cavities may consist of a free-piston coupler which can be balanced statically while being driven acoustically by the fluid pressure variations generated within the oscillator cavities. The piston coupler can be free to move dynamically as a rigid body to couple out to a load the acoustic forces generated by the push-pull cavity pressure variations without having to be subject to a net static force, arising from the elevated average pressure within the oscillator cavities and without having to resort to elastic extension. The piston coupler can transfer acoustic energy at high energy density from the oscillator cavities to a load or load COupling mechanism without being subject to fatigue under raidly varying strain. In some cases the piston coupler may cooperate with a mechanical or a fluid means in a part of the coupling means itself, which mechanical or fluid means introduces a compliance for maximizing power transfer between the generator and the load. In some cases too, the coupling means may develop a unidirectional biasing force as, for example, against a load in response to the elevated average pressures in the oscillator cavities rather than being exactly balanced statically.

As is more fully pointed out hereinafter, there are serious mechanical problems involved in phase synchronizing a plurality of self-excited acoustic vibration generators. Since each of the generators is self-excited, each transducer in an array can operate non-synchronously with respect to the rest of the generators in the array. This is a problem since, generally, synchronous operation is desired to enable a defined and stable acoustic beam pattern to be obtained, and to maximize acoustic loading on the individual generators.

Accordingly, it is an object of the present invention to provide novel means for combining a single device two or more acoustic vibration generators or oscillators having a common energy coupling member, whereby all generators aremaintained in phase synchronisrn.

Another object of the present invention is to provide an array of individual acoustic vibration generating devices, each of which is capable of transferring energy to a load in phase synchronism with one another.

atent O A still further object of the present invention is to provide an improved phase synchronism means for a plurality of acoustic vibration generators which is relatively simple to construct, inexpensive to manufacture, and economical of moving parts.

Other objects and advantages reside in certain novel features of arrangement and construction of parts which will become apparent from the following detailed description taken in connection with the accompanying drawings, wherein:

FIG. 1 is a central longitudinal sectional view, partly in elevation, of an acoustic vibration generator having a free-floating valve disposed in an equilibrium position within a portion of a housing of restricted inner diameter;

FIG. 2 is a transverse sectional View of the device of FIG. 1 taken along line 22;

FIG. 3 is a central longitudinal sectional view, showing the device of FIG. 1 with the valve displaced from the equilibrium position in one of two possible directions;

FIG. 4 is a central longitudinal sectional view of a device which differs from that of FIG. 1 in that the restricted inner diameter of the housing and the valve are enlarged so that the outer diameter of the valve is substantially equal to the maximum inner diameter of the generator housing;

FIG. 5 is a sectional view illustraing a cascade cor1 nection of a plurality of acoustic vibration generators, of the type exemplified in FIGS. 1-3, for supplying energy to a single load or energy receiver;

FIG. 6 is a sectional view, illustrating a modification of the generator of FIG. 1, wherein one of the oppositely disposed piston-type members terminates in a driving element of considerably different size than that of the other piston-type member and wherein a backing compliance is introduced;

FIG. 7 is a transverse sectional view of an acoustic vibration generator, taken along line 1910 of FIG. 8, which dilfers from that of FIG. 9 principally in that it includes a plurality of mechanical stiffeners;

FIG. 8 is a longitudinal sectional view of the acoustic vibration generator of FIG. 7;

FIG. 9 is a view, showing a pair of acoustic vibration generators, such as shown in FIGS. 6-8, arranged in accordance with the invention for radiation of acoustic energy in opposite directions;

FIG. 10 is a schematic diagram of an array according to the invention of interconnected acoustic vibration generators of the type shown, for example, in FIGS. 6-8, which provides for radiation of a beam of acoustic energy; and

FIG. 11 is a schematic diagram of a master-slave arrangement according to the invention of acoustic vibration generators such as exemplified in FIGS. 68.

Reference is first directed to FIGS. 1 to 3 which illustrate an acoustic vibration generator 1 equipped with a coupling means 2. The acoustic vibration generator 1 is of the push-pull type with a reflection path acoustic circuit, and is one of several types of applicable generators which are ideally suited for the coupling means to be described.

The acoustic vibration generator 1 of FIGS. 1 to 3 comprises a housing 11 having an axial bore 19, a stationary port structure 12 disposed centrally within this housing bore 19, a free-floating reciprocating valve 13 positioned within the stationary port structure 12, and a pair of freely movable piston-coupler members 20 and 21 which terminate opposite ends of the housing 11, and which are interconnected rigidly by shaft 70. The stationary port structure 12, free-floating reciprocating valve 13, and free piston assembly 29, 70, 21 divide the housing bore 19 axially into two acoustic chambers or cavities 15 and 16. In operation, these acoustic chambers 15 and 16 are to be fluid filled with a fluid medium under pressure. For that purpose, inlet ports 17 and 18 are located on the outer sides of the stationary port structure 12 for introducing fluid under pressure into the respective cavities 15 and 16. The coupling pistons 20 and 21 are provided with O-rings 22 and 23 which will permit free piston motion and yet which provide a hydraulic seal for the fluid within the cavities 15 and 16.

The stationary port structure 12 cooperates with valve 13 to form variable annular discharge orifices 36 and 37. The orifice 36 is formed by the upper edge 42 of land 40 of the stationary port structure, and by the lower edge 43 of rim 14 of the valve 13. Likewise, the orifice 37 is formed by the lower edge 46 of land 41 of the stationary port structure and by the upper edge 47 of rim 14 of valve 13. The free-floating valve 13 is free to move axially within the land region of the stationary port structure in response to fluid pressure variations developed within acoustic cavities 15 and 16, thereby to control the flow of the fluid medium from the respective cavities 15 and 16 into the discharge or exhaust cavity 32. The fluid medium is passing through the variable annular orifices 36 and 37 enters the discharge cavity 32 at a reduced pressure. The fluid medium leaves the discharge cavity 32 by way of exhaust port 51) and exhaust channel 51.

The complete hydraulic path of the fluid medium in the acoustic vibration generator 1, shown in FIGS. 1-3, may be traced by starting at inlet channel 25, in which a pressurized fluid medium, such as a low viscosity hydraulic oil or other suitable fluid medium, is introduced. The inlet channel 25 diverges into lower and upper channels 26 and 27, respectively, terminating at inlet ports 17 and 18. The length of the upper channel 27, as well as that of the lower channel 26, may be chosen to be a quarter-wavelength of the operating frequency of the generator 1 in order to provide a filtering action to isolate the acoustic circuit of the generator 1 from the flow source 55. The flow source 55 may be a conventional hydraulic pump. From the inlet ports 17 and 18, the fluid medium issues into respective acoustic cavities 15 and 16. Upon filling these cavities, the fluid medium then passes through variable area orifices 36 and 37 into the discharge cavity 32, through the exhaust port 50 into the exhaust channel 51 and thence to the low pressure side of flow source 55.

The free-floating valve 13, as Well as the acoustic cavities 15 and 16, the coupler pistons 20 and 21, and stationary port structure 12, are normally cylindrical in cross-section, as shown in FIG. 2. The free-floating valve 13, as such, forms no part of this invention since it is disclosed in the copeuding application heretofore mentioned. The valve 13 comprises a cylindrical body 9, and may have radial fins 57 projecting one the end faces 61 and 62. The fins 57 are recessed from the outer diameter of the valve body 9 so as not to restrict the annular orifices 36 and 37. These fins may be used to give the valve 13 a spinning and centering action when driven by means of momentum transferred from the rotating fluid medium in the acoustic cavities 15 and 16. The rotation of the fluid medium is generated by directing the fluid flow towards one side of the acoustic cavities 15 and 16 as it enters through the inlet ports 17 and 18, respectively. This tangential entrance of the fluid is particularly evident from FIG. 2.

The operation of the coupling arrangement for the acoustic vibration generator 1, as shown in FIGS. 1 to 3, may be better understood if the operation of acoustic vibration generators discussed in United States Letters Patent No. 2,792,804 is reviewed. Consider that a steady flow of fluid under pressure from an external source is established through the acoustic generator when the freefloating valve 13 is in the equilibrium position, i.e., in a position such that the pressure in both cavities 15 and 16 are equal, as shown in FIG. 1. In such a quiescent state, the pressure drop across the annular orifices 36 and 37 of the stationary port 12 is equal and there is no net thrust tending to displace the position of the valve 13 or the coupling pistons 20 and 21. The rod-like connecting means 70, in accordance with this invention, by supporting the equal but oppositely directed average forces on pistons 20 and 21, maintains the coupling piston structure statically balanced. If the valve 13 should now be subjected to an incremental movement from its equilibrium position, such as might derive from turbulent flow in the orifice region, resulting at some instant, for example, in an incremental displacement of the valve towards cavity 15, as shown in FIG. 3, there will result a deceleration of the fluid flow through acoustic cavity 15 accompanied by an increase of local pressure in acoustic cavity 15, while in acoustic cavity 16 there will result an acceleration of the fluid flow accompanied by a reduction of local pressure.

The higher instantaneous pressure in cavity 15 and the lower instantaneous pressure in cavity 16 exerts a net force increment on the valve 13 directed opposite to the above assumed incremental displacement of valve 13. This oppositely directed incremental force will only tend to increase the above assumed displacement. This result derives from the fact that a mass, such as valve 13, when subjected to an oscillatory force field, will have its acceleration in phase with the applied force while its displacement will be out of phase with the applied force. An incremental displacement of the valve 13 thus will be accompanied by asymmetrical or push-pull pressure changes in cavities 15 and 16 reacting on the valve 13 to accentuate the given displacement and, consequently, the driving force exerted on valve 13. This above statement illustrates the regenerative nature of the valving action. If the losses in the acoustic circuit of the generator are not too high, this regenerative feature will allow oscillations to build up and be sustained at a frequency for which the acoustic reactance, seen looking into the cavities 15 and 1-6 from the orifices 36 and 37, is equal but of opposite sign to the acoustic reactance presented by the acoustic cavities 15 and 16 depends not only upon the elastic compliance of the contained fluid but also upon the impedance presented to these cavities by the output coupling pistons 20 and 21.

At the oscillation frequency the orifices 36 and 37 look into a resistive load which is made up in part by the internal losses of the system and in part 'by the external load presented to pistons 20 and 21. It is evident that, for efficient operation, the load presented by internal losses should be a small fraction of the external load.

Oscillations will be sustained at an oscillation amplitude for which the positive acoustic resistance seen by the orifices equals the negative resistance presented by the flow modulated by the valving action. As the fluid flow through the orifices 36 and 37 is a non-linear function of orifice pressure differential and orifice area, a negative resistance can generally be obtained for some value of supply pressure and for a given load resistance for which oscillations will commence and stabilize at an optimum design amplitude. The resulting stable asymmetric pressure variations developed within the generator cavities then operate on interconnected pistons 20 and 21 to deliver a net acoustic force against some applied acoustic load.

In summary then, the generation of acoustic energy by generator 1 and the transfer of a portion of this energy to an external load is accomplished by a regenerative valving action which generates a self-sustaining oscillation of the free-floating valve 13, thereby modulating asymmetrically the flow through orifices 36 and 37 and causing push-pull pressure variations to exist in cavities 15 and 16. These push-pull pressure variations not only operate on freely moving pistons 20 and 21 to move dynamically the rigid coupling means 2 comprising elements 20, 70, 21 to convey energy to an external load, but in addition, react back on the exposed faces of valve 13 to sustain the action of the generator. Under optimum design conditions, a major portion of the acoustic energy generated by the valving action is transferred through the motion of coupling pistons 20 and 21 to an external energy receiver while the remaining portion of the generated energy is reflected by coupling pistons 20 and 21 to sustain the oscillation of the free-floating valve 13.

FIG. 4 illustrates an acoustic vibration generator in which the stationary port structure 12 and the valve 13 have been designed such that the cross-sectional area of the valve 13 is substantially equal to the radial crosssectional area of the bore of housing 11 in order to prevent a net axial force from being exerted upon the housing. The elements in FIG. 4 corresponding to those of FIGS. 1 to 3 are designated by similar reference numerals. The acoustic generator 1 of FIG. 4, like that of FIGS. 1 to 3, includes a housing 11, a stationary port structure 12, a free-floating valve 13, acoustic cavities or chambers and 16, inlet channels 26 and 27 terminating in respective inlet ports 17 and 18, and an exhaust port 50 at one end of an exhaust channel 51. The radial fins 57 of FIGS. 1-3 have been modified slightly and are generally designated as radial fins 57a in FIG. 4. The radial fins 57 and 57a do not form any part of this invention. The port structure 12 is formed by providing a recess 32 in the cylindrical housing 11. Orifices 36 and 37 are formed in the space between the outer rims 43 and 47 of valve 13 and the projecting rims 45 and 44 of the housing 11, respectively at the corresponding extremities of recess 32. The recess 32 serves as a discharge or exhaust port through which the fluid flows in returning by way of the exhaust port 15 and exhaust channel 51 to the low pressure side of the fluid pump.

With the arrangement shown in FIG. 4, the axial forces developed in the cavities 15 and 16 during modulation of the fluid flow by the valve 13, will not be exerted upon housing 11, as in the case of the device of FIGS. 1 to 3, since the net applied force on the moving coupling members (pistons and 21) is exactly balanced by the inertial reaction of valve 13. This feature, however, may be used with any of the devices described in this application.

It is possible to apply the techniques of the present invention to a plurality of acoustic vibration generators in either a series or parallel array. FIG. 5 discloses a plurality of serially-connected and concentrically aligned acoustic vibration generators. Two complete generators, namely, generators 201 and 202, are shown in FIG. 5; each of these generators is similar to the generator shown in FIGS. 1 to 3. The coupling means 250 for generator 201 includes coupling pistons 203 and 204 and rigid connecting means 206; while the coupling means 251 of generator 202 includes pistons 204 and 205 and rigid connecting means 207. The serial connection of the coupling means 250 and 251 of the two generators is achieved through coupling piston 204 which is common to the two generators. Portions of two other generators 210 and 212 are shown at either end of generators 201 and 202 and may be used in conjunction with the latter generators, if required. Any number of generators can be connected in tandem, as long as the composite coupling structure can move at the oscillation frequency substantially as a rigid body. An inlet channel 240 conveys pressurized fluid to the acoustic generators 201 and 202 and an exhaust cavity 241 provides a return passage for the exhaust fluid medium.

The serially-connected acoustic vibration generators 201 and 202 and the corresponding composite coupling arrangement may be preferred in some instances in that more power can be concentrated in a smaller crosssectional area and also because the acoustic vibration generator combination possesses an inherent and simple capability of synchronization.

The operation of the serially-connected acoustic generators 201 and 202, as shown in FIG. 5, is generally similar to the operation of a single acoustic generator. The valves 231 and 232 may oscillate at frequencies for which the acoustic reactance of a given valve equals but is of opposite sign to the acoustic reactance seen looking at the generator cavities 215 and 218 from annular orifices 236 and 237 of the respective generators 201 and 202. As in the generator 1 of FIGS. 1 to 4, the coupling pistons 204, 205 and 206 reflect back a portion of the acoustic energy to the cavities 215, 216, 217 and 218 to sustain the valves 231 and 232 in oscillation, thereby keeping the acoustic generators 201 and 202 operative.

So long as the acoustic constants of both generators 201 and 202 are essentially identical, the interchange of acoustic energy between them to coupling pistons 203, 204 and 205 will tend to lock them in at the same frequency. However, two basic modes for the two generators 201 and 202 exist. One mode is that for which the valves 231 and 232 move in phase opposition. In this case, the net thrust developed on piston 203 and the left side of piston 204 by virtue of pressure variations in cavities 215 and 216 is equal but of opposite sign to the net force exerted on piston 205 and the right side of piston 204. As a result, the net force exerted on the composite piston structure is Zero and no acoustic energy is coupled out of the composite generator system. The other basic mode is that for which the valves 231 and 232 move in phase synchronism, giving rise to pressure variations in the corresponding cavities of the two generators 201 and 202 which exert in-phase forces on the corresponding faces of the composite piston coupling structure; in this mode, the sum of the forces developed in each of the cavities add to develop a net thrust on the coupling pistons to transfer acoustic energy to an external energy receiver.

In order that the mode for which the valves 231 and 232 move in phase may be naturally selected over the mode for which the coupling arrangement does not move, means may be provided for short circuiting the in-phase pressure variations in cavities 216 and 217 at the frequency of the undesired mode. This means may take the form, for example, of a fluid channel 238 interconnecting the cavities 216 and 217 of generators 201 and 202, respectively, as shown in FIG. 5. The length of channel 238 between cavities 216 and 217 in which the in-phase pressure variations are to be avoided may be equal to odd multiples of a half-wavelength for compressional wave transmission along the channel 238 at the frequency of the undesired mode. So long as the two possible modes are not widely separated in frequency, the above specifide channel 238 will yield an inlet impedance as seen by the respective cavities 216 and 217 which will be low at the undesired mode, thereby suppressing the latter mode, and high at the desired mode, thereby not appreciably affecting it.

Once synchronization has been established between the two acoustic generators 201 and 202, the coupling pistons 203, 204 and 205 and connecting means 206 and 207 will deliver acoustic power to an energy receiver from generators 201 and 202 equal to the algebraic sum of the available acoustic power generated Within the acoustic cavities 215 and 216 of generator 201 and the available acoustic power generated within the acoustic cavities 217 and 218 of generator 202. The energy receiver may, for example, be a pile driver, drill or vibrating attachment for processing foods, chemicals or metals.

Referring next to FIG. 6, an acoustic vibrator generator 1 is shown which incorporates versatile means for achieving simultaneously an optimum impedance match as well as maximum power transfer between the oscillator power converting orifices and an external load. In addition, the structure is so designed as to provide, if desired, a biasing force to maintain a fixed average position of the coupling means, as the latter may, in some modifications, be normally free of mechanical constraint. The biasing force can also help to minimize a possible internal cavitation problem.

The oscillator of FIG. 6 is identical in many respects with the oscillator of FIG. 1. The principal differences which give rise to the above-named impedance and energy transfer characteristics result from the introduction of an area transformation between the driving pistons and the radiating piston member, and the inclusion of a backing compliance to resonate with the mass of the aggregate piston coupling structure.

The oscillator includes a more-or-less cup-shaped hous ing 11 having one end of a bore threaded for fixedly attaching a cylindrical block 101 whose outer diameter is substantially equal to the inner diameter of the bore in housing 11. The housing 11 and block 101 are provided with suitable passages for the flow of fluid. In addition, block 101 is provided with a stationary port structure 12, valve 13, pistons 20 and 21 and connecting means 70, together with cavities 15 and 16, as in FIGS. 1 to 4. The block 101 includes cylindrical bores within which the pistons and 21 may move. Housing 11 also contains a cylindrical enclosure or chamber 102 to act as a termination to piston 22. This chamber 102 may be filled with a suitable fluid to serve either as a pressure release or fluid complaint termination, as desired.

The hydraulic circuit of the oscillator of FIG. 6 may be traced as follows. Hydraulic fluid under pressure enters at inlet to flow through two lines 266 and 27 to issue at ports 17 and 18 into oscillator cavities 15 and 16. The length of these feed lines may be of the order of a quarter wavelength in the contained fluid at the oscillation frequency in order to isolate the acoustic circuit of the oscillator from the steady flow source.

The fluid, upon filling cavities 15 and 16, issues through annular variable area orifices 36 and 37, formed between the outer rims of annular valve 13 and the rims of stationary port structure 12, to discharge into cavity 32. The discharge cavity 32 is connected to output port 50 in channel 51 from which the fluid returns to the source.

The valve 13 is free to move back and forth along coupling 70 which joins rigidly the two piston couplers 20 and 21. During oscillation, the valve 13 will alternately open and close orifices 36 and 37. It has been mentioned that with a massive valve 13 of the configuration illustrated in FIG. 1, motion of the valve at the oscillation frequency will be accompanied by pressure variations in cavities 15 and 16, arising from the modulation of the flow through orifices 36 and 37, which tend to sustain the valve motion. The oscillation frequency can again be shown to be that for which the acoustic mass reactance of the valve equals the net stiffness reactance seen looking into cavities 15 and 16.

If we assume that an oscillation has commenced in the acoustic circuit of the device of FIG. 6, then as the valve moves to the left-hand cavity 15 the instantaneous pressure therein will be increasing while the instantaneous pressure in the right-hand cavity 16 will be decreasing. Such pressure variations about the average supply pressure will tend to operate on drive pistons 20 and 21 to exert a force on these pistons in a push-pull manner. As drive pistons 20 and 21 are assumed to be coupled rigidly by interconnecting rods 70, the arithmetic sum of the acoustic forces operative on pistons 20 and 21 will therr drive the composite piston coupling means, including the enlarged portion 121 of piston 21, otherwise referred to as a radiating element, resulting in motion of the output coupling face 90. A hydraulic seal is maintained between the radiating element 121 and the housing 11 by an O- ring 98. The motion of the output coupling face 90 transfers acoustic energy from the oscillator to an external load, such as a fluid medium in contact with face 90.

As a result of the reduction in area between face and the drive faces of pistons 20 and 21 in the device of FIG. 6, acoustic energy is coupled from the oscillatorcavities 15 and 16 at high energy density effective over a relatively small drive area to the external load at a lower energy density extending over a larger radiating area. This direction of area transformation is useful when coupling acoustic energy from the oscillator to open bodies of fluid where it is desirable not only to avoid cavitation at the transducer face but also to achieve a high value of radiation resistance for efficient operation.

The cavity 91 of FIG. 6 behind the output coupling piston 121 is shown fluid-filled and surround by solid walls exhibiting high rigidity. This fluid may be the same hydraulic oil employed as the oscillator fluid or possibly a high compliance silicone oil. The fluid in cavity 91 acts as a compliant backing reactance to cancel a portion, if not all, of the mass reactance presented by the composite piston coupler.

For a given piston coupler and radiation load, the load impedance presented to the oscillator cavities will depend upon the relationshipbetween the resonant frequency of the piston coupler mass and the backing cavity compliance and the nominal oscillation frequency. The impedance presented by the load to the oscillator cavities can thus be define'd only upon specifying in combination the area of piston face 90, the value of the radiation load, the mass of the composite piston, the mass and driving area of the valve 13, the area of the drive pistons 20 and 21, and the volume and fluid characteristics of both the oscillator cavities 15 and 16 and the fluid backing cavity 91. This large number of interrelated design factors generally permits one to obtain with reasonable case not only an optimum impedance match but also a condition of maximum power transfer between the load and the oscillator source for a given power level.

It can be readily shown that the condition of maximum power transfer will occur when the resonant frequency of the piston mass and backing compliance is equal to the oscillation frequency. In order for the oscillation frequency to coincide with the resonant frequency of the loaded coupling structure, the following relationships must apply.

In Equation 1, R is the acoustic resistance (ratio of acoustic pressure to acoustic volume velocity) presented at resonance by the driving pistons 20 and 21 to the acoustic cavities 15 and 16. R derives from the resistive portion of the load impedance presented to the face 90 of radiating piston 121, approximately modified by the area transformation between piston face 90 and the drive pistons 20 and 21. M and M are, respectively, the acoustic masses, referred to the drive cavities 15 and v16, of the composite coupling piston 20, 70, 21, 121 and of the valve 13. C and C are, respectively, the acoustic compliance of the fluid contained in drive cavities 15 and 16, and the compliance, as referred to these cavities, of the elastic element 91 backingv the radiating piston member 121. The quantity wo is defined as the radian resonance frequency of the acoustic mass of the valve with theacoustic compliance of the fluid contained within cavities 15 and 16; we is also defined to the simultaneously the resonance frequency of the coupling piston mass with its backing compliance.

The oscillator frequency will coincide with wO when Equations 1 and 2 are satisfied. In these circumstances, the condition of maximum power transfer between the oscillator orifice and the radiation load is achieved for any given acoustic driving pressure in the oscillator cavities. On the other hand, if, for example, Equation 2 is satisfied while Equation 1 is not, i.e., if

2 D/IAP=D/IAV R AL CAO CAP then it can be shown that two oscillation frequencies are possible neither one corresponding to am, one being lower than we, and the other being higher than (.00. In these circumstances the condition of maximum power transfer is not obtained, and, in fact, the oscillator can be shown to select that mode for which a relative minimum of power transfer will occur. The condition of Equation 3 is therefore generally undesirable, and should be avoided whenever possible.

Although in the above discussion the drive areas of pistons and 21 have been assumed implicity to be equal, they need not be, and, in fact, are so illustrated in FIG. 6. Thus in contrast with the' device of FIGS. 1 and 4, the piston coupler assembly of FIG. 6 is not necessarily balanced statically, and can exhibit, for example, a net static thrust directed toward the rear of back cavity 91. Cavity 91 is filled, as before stated, with a resilient fluid which may be inserted through an inlet 93 and sealed off by plug 92. Since cavity 91 is fluid filled and sealed by an O-ring 98, the static pressure in cavity 91 will rise in reponse to the thrust on the piston until a balance of forces is achieved. This static bias of the piston against back gravity 91 is useful to avoid dynamic cavitation in cavity 91, and also to establish a positive equilibrium position of an otherwise free piston.

It is to be noted that, in accordance with a main feature of the invention, the generator of FIG. 6 does not require the bending or extension of any solid mechanical member in order to perform in the intended manner. In fact, the energy coupling structure of the generator preferably moves as a rigid body, and where compliant members are needed to satisfy operational requirements fluid compliant elements are employed. These fluid elements exhibit a distinct advantage over mechanically compliant structures in that they are not subject to permanent fatigue failure under repeated stress.

Mechanically compliant elements are useful in some instances, however, and an example of their use is illustrated in FIGS. 7 and 8. The principle of operation of this generator is essentially identical with the devices of FIG. 6. However, cavity 91 is intended, in this stance, to be filled with gas rather than with liquid. The desired backing stiffness for piston 121 is now obtained, not from the fluid in cavity 91, but by a plurality of mechanical stiffeners or studs 95.

In the acoustic vibration generator 1 of FIGS. 7 and 8, the cylindrical block 101 includes, in addition to a hydraulic circuit, a plurality of bores 103 and 103' for receiving mechanical studs 95 and bolts 106, respectively. Each of the studs 95 consists of a portion 97 having an integral enlarged portion 96 at one end which rests against a shoulder 107 in bore 103. The other end of the stud 95 remote from the head 96 threadably engages the enlarged portion 121 of piston 21; a nut 108 rests against the back wall 109 of the enlarged portion 121 of piston 21. The bolt 106 includes a shank 99 threaded into the head portion 96 of stud 95 as well as a head 104 which bears against the shoulder 105 in bore 103'. The bolt 106 serves as a means for supporting the studs 95.

The piston coupler 20, 70, 21 thus drives radiating element 121 which, in turn, is supported mechanically from the housing 11. The resonant frequency of the mass of element 121 and its fluid load with the stiffness of the studs 95 is normally made coincident, as mentioned before with reference to Equations 1 to 3, with the natural oscillation frequency of the oscillator. In these circumstances, maximum power transfer between the oscillator orifices and the external load will result.

The choice of mechanical stiffness members 95 over a liquid spring, such as the liquid-filled cavity of FIG.

6, may be dictated by considerations of cavitation. In the event that the oscillator is to operate at ambient pressures near atmospheric pressure, signal pressures in liquid-filled cavity 91 may be suflicient to cavitate the contents of that cavity. In this case, it is desirable to replace the liquid spring by a mechanical equivalent and to fill cavity 91 with gas. On the other hand, if the oscillator is to be operated at suflicient su'bmergence depths, where cavitation can be suppressed, then the liquid spring may be desirable. In particular, for applications at great submergence depths, where hydrostatic pressures are high, the liquid spring exhibits substantial advantages. The free piston in the device of FIG. 6 automatically adjusts its position so that the hydrostatic forces acting on the front and back faces of element 121 are equal. In addition, the liquid in cavity 91 of FIG. 6 forming the spring is not subject to compressional fatigue, as would be the case with the mechanical studs 95 of FIG. 8, if the latter were to support the static hydrostatic force in addition to the dynamic movement of radiating element 121. Consequently, with a liquid compliant backing to the piston radiator 121, there is no practical limit to submergence depths.

In FIG. 9, two acoustic vibration generators 1a and 1b, which are of the type shown in FIG. 6, are arranged back-to-back with an acoustic transmission line 300 passing through the respective housings 11 and 11 interconnecting the cylindrical enclosure or cavities 102 and 102 of the generators 1a and 1b, respectively. The light arrows indicate the direction of flow of fluid into the respective inlet channels 25 and 25 and out from the respective exhaust channels 51 and 51' of the generators 1a and 1b. The heavy arrows indicate the general direction of propagation of acoustic energy from respective radiating faces and 90'. The length of the line 300 should be of the order of wherein n is zero or an integer and A is the wavelength in the contained fluid at the operating frequency of the device, if it is desired that the pistons of the two generators move in phase (i.e., in opposite directions in the device of FIG. 9). The actual length of the line 300 may, in practical cases, necessarily depart slightly from because of the effect of the compilance of the cavities 102 or 102' upon the line 300.

The linear spacing between the radiating faces 90 and 90' of the generators in and 1b may 'be chosen to provide optimum radiation loading on the pistons of the two generators. Furthermore, the housing of the two generators may be physically separated rather than contacting one another at the rear end of each housing. It should be noted also that the two generators may be formed from one integral structure or housing 11 instead of two separate housings 11 and 11'. In either case, acoustic coupling between the two cavities may be achieved by suitable passages extending from the cylindrical cavities or enclosures 102 and 102' of the single housing, which passages are interconnected by appropriate external connectors to the external transmission line 300. Inasmuch as the inlet orts of the fluid inlet channels 25 and 25' are displaced a half wavelength when the fluid inlet channels are a quarter wavelength long, the inlet ports of the two devices then may be cross-connected by two acoustic trasmission lines to achieve interconnection of the two generators.

In some instances, it may be desirable to arrange two or more generators in a planar array so that the acoustic energy radiated from each re-enforces that of the others to provide a particular beam pattern. Such an arrangement is indicated in FIG. 10 wherein three generators,

1a, 1b and 10, which are of the type shown in FIG. 6, are interconnected by acoustic transmission lines 310, 320 and 330. The general direction of energy propagation from the generators of FIG. is indicated by the arrows. For simplicity, fluid connections to the generators have been omitted in this figure. The length of each of the lines may again be of the order of where 7t is the wavelength of the generated signal in the contained fluid and n is either zero or an integer, thereby causing the radiating elements to be driven in phase synchronism.

A master-slave system of acoustic vibration generators is shown schematically in FIG 11 wherein a master or control acoustic vibration generator 4111 described and shown in US. Letters Patent No. 2,792,804, particularly in FIG. 4 therein, which patent issued to John V. Bouyoucos May 27, 1957, has as its only load acoustic transmission lines 450a, 4501; 45011 feeding respective slave or controlled acoustic vibration generators 501a, 501b, 501n (of the type shown in FIG. 6) which, in turn, may couple energy to a radiation load. The manner of connection of the master generator to the slave generators is similar to that shown in FIGS. 9 and 10. Each of the acoustic transmission lines is of such length as to achieve appropriate coupling of energy from the master generator to the slave generators and to interlock the slave generators in a desired phase relationship. Although each generator may be a non-directional source by itself, the combination of generators may yield a directional array, as is also the case in the array of FIG. 10. By controlling appropriately the length of the individual transmission lines 450a, 4S0b 450m the directed acoustic beam generated by the array may be steered to any angle from broadside to end fire.

What is claimed is:

1. A plurality of acoustic vibration generators, each comprising a source of pressurized fluid, a housing having formed therein a pair of chambers connected to said source containing a fluid under pressure, a fluid control valve means disposed in the housing between said chambers movable in response to fluid pressure variations within said chambers to control the flow of fluid through said chambers, rigid coupling means including a rigid coupling member for each of said chambers freely movable within said housing and acoustically driven by the pressure variation in the fluid within said chambers, said housing furend of the said coupling means, the other end of said coupling means terminating in an enlarged radiating element. and acoustic energy transmisison means interconnecting said enclosures of said generators to form an array of radiating elements.

2. A plurality of acoustic vibration generators each comprising a source of pressurized fluid, a housing having formed therein a pair of chambers connected to said source containing a fluid under pressure, a fluid control valve means disposed in the housing between said chambers movable in response to fluid pressure variations within said chambers to control the flow of fluid through said chambers, rigid coupling means including a rigid coupling member for each of said chambers freely movable within said housing and acoustically driven by the pressure variation in the fluid within said chambers, said housing further containing a fluid-filled enclosure terminating at one end of said coupling means, the other end of said coupling means terminating in an enlarged radiating element, and acoustic energy transmission means interconnecting said enclosures of said generators to form an array of radiating elements, said transmission means having a length substantially equal to where n is zero or any integer and A is the wavelength at the operating frequency of said generators.

References Cited UNITED STATES PATENTS 2,792,804 5/1957 Bouyoucos et a1 116-137 2,859,726 11/1958 Bouyoucos et al. 116-137 3,004,512 10/1961 Bouyoucos et al 116-137 3,105,460 10/1963 Bouyoucos et al 116-137 3,143,999 8/1964 Bouyoucos et a1 116-137 3,212,472 10/ 1965 Bouyoucos et a1 116-137 3,212,473 10/1965 Bouyoucos et al 116-137 LOUIS J. CAPOZI, Primary Examiner 

